"Surge" is an unstable impeller condition with potentially destructive consequences in a centrifugal compressor. As an example, if a typical turbine engine has one occurrence of surge, the engine is removed from service, overhauled, and the impellers replaced.
A centrifugal compressor with fixed vanes operating at: 1) a fixed RPM; 2) a fixed pressure ratio P.sub.O .backslash.P.sub.I (output pressure/input pressure); and 3) a fixed vapor mass flow rate, can go into surge with a sudden change in evaporator load, the evaporator load being directly related to the refrigerant flow rate. This phenomenon occurs when the evaporator blower speed is reduced or when the evaporator inlet air enthalpy level drops quickly when changing from outside air to recirculated air. The phenomenon is illustrated in FIG. 1, a chart illustrating general compressor performance characteristics with the vertical axis representing pressure ratio (P.sub.O /P.sub.I) and the horizontal axis representing refrigerant flow rate (M).
Point A in FIG. 1 represents a compressor operating at refrigerant flow rate M.sub.1, pressure flow rate PR.sub.1 and a speed of N=100% (of rated compressor RPM) Assume an ambient temperature of T.sub.1 =100.degree. F., an ambient humidity level of H.sub.1 =40% relative humidity, and an enthalpy level of E.sub.1 =42.7 BTU/lb. If the evaporator inlet air enthalpy decreases from E.sub.1 to E.sub.2 (defined by P.sub.2 H.sub.2 =75.degree. F., 48% relative humidity; enthalpy of 27.9 BTU/hr. from a psychometric chart) by changing from outside air to recirculated air, the compressor will go into surge by shifting from point A to point B where the refrigerant mass flow rate is reduced from M.sub.1 to M.sub.2, where M.sub.2 is the new flow rate required to satisfy the evaporator. In this example of a surge condition, if the compressor maintains a constant pressure ratio PR.sub.1, the compressor speed is reduced from N=100 to N=86%. This causes the compressor to run under the conditions illustrated at point B in FIG. 1, in which case the compressor is running at the surge line, and exceeding surge margins SL.sub.norm and SL.sub.max at flow rate M.sub.2.
Another surge condition would exist if the compressor begins at the conditions indicated at point A, and the compressor speed is maintained constant at N=100% and the pressure ratio is increased until the compressor exceeds the surge limits and the surge line. In this case, the mass flow value associated with the N=100% compressor RPM line is significantly greater than the desired M.sub.2 flow rate at the surge line. This point is restricted by surge margin limit SL.sub.max at point A'. The refrigerant flow rate at A' is higher than the desired M.sub.2 flow rate.
"Surge margins" are the difference between the compressor surge line and an actual acceptable operating condition at the same flow rate. The compressor pressure ratio and the compressor speed are reduced at the same flow rate but with a surge margin reduced for a constant compressor speed line. FIG. 1 illustrates the surge line and two surge margin lines (SL.sub.norm, SL.sub.max) The greater the difference, the less chance there is of encountering surge during transient conditions. The surge margins are defined as follows: a) SL.sub.norm is the normal surge limit and is a slow response curve; and b) SL.sub.max is the fast response surge limit and is optional. The slow response surge margin values are based on flow rate change values with time constants and magnitudes substantially less than the fast change values. The actual magnitude of the surge margin line relative to the surge line is dependent on the individual system. The SL.sub.norm surge margin is used during stabilized system operation where there are small changes in inlet enthalpy occurring over a relatively long period of time. The SL.sub.max surge margin is enacted during sudden large shifts in evaporator capacity over a very short period of time, e.g., changing the evaporator blower speed or changing from outside air to recirculated air. The reason for the SL.sub.max surge line is to provide additional surge protection during large, short duration system load changes.
Traditionally, turbine-based machines utilize a complex mechanically regulated vapor bypass which controls vapor flow through the compressor in order to permit such transitions to occur without going into surge. This results in a complex system of valves and ports that increase the size, weight, and cost of a centrifugal machine.
Another means of controlling surge in centrifugal compressors is described in Kountz et al., U.S. Pat. No. 4,546,618. This patent describes the use of variable inlet vanes, commonly called pre-rotational vanes. These vanes require a driver motor, complex mechanical hardware, and pivotable turning vanes that must operate in high gas flow areas over a wide range of temperatures with a high degree of accuracy. The device is microprocessor-controlled, but provides only an iterative process for avoiding surge, and does not provide predetermined paths for operating at optimized efficiency.
It is desirable to provide a surge control system which does not require such complex vapor bypass controls or mechanically variable vanes for preventing surge.